The need for driers that can effectively and efficiently reduce the moisture content of finely divided materials is well known. One problem commonly encountered in drying such materials is clumping or agglomeration. Some prior art methods and apparatus have utilized rotating shafts and stirring arms to stir particulate matter during drying. Others have utilized hot gas injected under high pressure to fluidize a bed of particulate matter during drying. Still others have employed stirring devices in combination with the injection of hot gas to effect drying. However, problems are still encountered with the conventional, commercially available driers, and their methods of use.
Such problems include, for example, packing of moist particulate matter by the rotating stirring arms; channeling that leads to incomplete drying, agglomeration and spot overheating; carryover of entrained solids from driers into filters and overhead recovery systems; inability to effectively remove undesirable reaction products; excessive energy use; and the like.
Drying is often required in order to satisfactorily prepare amorphous or crystalline polymers for subsequent molding or extrusion operations. For example, polyethylene terephthalate (PET) pellets used in molding carbonated beverage bottles is typically dried prior to injection molding the parisons from which the bottles are later blown.
Conventional methods and apparatus for the aftercondensation of amorphous polymers are disclosed in U.S. Pat. No. 4,585,366, and the references cited therein. Two primary problems previously encountered in the aftercondensation of amorphous polymers, and polyesters in particular, are agglutination and the removal of undesirable gaseous reaction products such as acetaldehydes. The removal of aldehydes from a static bed of crystalline PET by forced air or inert gas is specifically addressed in U.S. Pat. No. 4,230,819.
In previously disclosed methods and systems designed to overcome these two primary problems, other problems have been encountered which have also limited their usefulness and efficiency. Thus, rotary drum driers have been employed to limit agglutination, but their effectiveness has been hampered by dust formation, high energy consumption, and the need for operating in a batch rather than continuous process.
The use of simple shafts or columns as drying vessels has resulted in agglomeration of the granulate in the upper portions of such vessels, necessitating the use of crushing or peeling devices in the discharge section to break up the clumps, which again boosts energy consumption and causes dust formation.
To avoid such problems, others have employed fluidized beds for precrystallizing the polymer prior to recondensation. With fluidized beds, however, it is difficult to control residence time of the particles. Energy requirements are also great, and separate vessels are required for each step of the two-stage process.
In U.S. Pat. No. 4,584,366, the inventors disclose an apparatus comprising a single vessel with two stacked sections. The upper section contains an agitator preferably comprising a rotary shaft having flat stirring bars appended thereto. The lower section, into which the polymer flows under gravitational force from the upper section, is merely a cylindrical vessel through which the polymer flows without separation or interruption. Hot inert gas is injected into the lower portion of each section, and flows upwardly through the polymer bed to a collection system where it is filtered, dried, reheated and recirculated to the vessel. However, the apparatus disclosed in U.S. Pat. No. 4,584,366 requires a residence time of 5 hours to adequately dry and recondense PET granulate.
A polymer drier, drier system and method of operation are therefore needed that will permit finely divided particulate matter, and more particularly, polymer materials such as PET pellets or granulate to be dried in a much shorter time than five hours, thereby achieving correspondingly greater throughput. This advantage should desirably be attained without increasing the required capital investment and operating expenses to such an extent that the resultant advantage in throughput is more than offset by these other factors. Also, the advantage in throughput must be achieved without sacrificing the physical properties of the product. With PET granulate intended for use in food grade applications, for example, this means that the acetaldehyde level of the recondensed polymer should be maintained below a specified maximum level. Also, moldability and structural integrity of the products to be molded from the recondensed polymer may require that the intrinsic viscosity of the dried and recondensed polymer be maintained within a specified acceptable range.
According to the present invention, a drier, drier system and method are provided that achieve all of the foregoing advantages without any associated disadvantages that would negate the benefits to be achieved through use of the invention.